John McCain has declared victory, addressing supporters at 9:45 p.m. in Texas.

"I am very grateful for the broad support you have given our campaign," McCain said. "And I am very, very grateful and pleased to note that tonight, my friends, we have won enough delegates to claim with confidence, humility and a sense of great responsibility that I will be the Republican nominee for President of the United States."

"I want to thank all of you here -- all the Republicans, independents, and independent-thinking Democrats, in all parts of the country, who supported our campaign for the nomination, and have brought us across the finish line first, an accomplishment that once seemed to more than a few doubters unlikely."

McCain also said that his campaign was beginning its most important phrase: "to make a respectful, determined and convincing case to the American people that our campaign and my election as President, given the alternatives presented by our friends in the other party, are in the best interests of the country we love."

Former Arkansas Gov. Mike Huckabee has withdrawn from the Republican presidential race, telling supporters at about 9:15 p.m. eastern time that it "looks pretty apparent tonight" that Arizona Sen. John McCain will reach the 1,191-delegate threshold needed to clinch the GOP nomination, as the networks projected a little earlier this evening.

Huckabee said he called McCain to congratulate him and offered "my commitment to him and to the party to do everything possible to unite our party, to more importantly to unite our country, so that we can be the best nation we can be."

"Senator McCain has an honorable campaign because he is an honorable man," Huckabee said.

Huckabee noted that he began his campaign with little name recognition and financial resources. With a shoestring budget, Huckabee won a handful of Southern primaries in the Super Tuesday voting on February 5, which established McCain emerged as the clear Republican front-runner and effectively forced the exit of former Massachusetts Gov. Mitt Romney, who had been competing with Huckabee for the votes of conservatives.

Dallas County (Dallas) -- 123,260 votes, Obama 66%, Clinton 34%

Tarrant County (Fort Worth) -- 81,816 votes, Obama 59%, Clinton 40%

Hidalgo County (McAllen; 88 percent Hispanic) -- about 51,000 votes, Clinton 74%, Obama 25%

Webb County (Laredo; 94 percent Hispanic) -- 27,630 votes, Clinton 77%, Obama 21%

El Paso County (El Paso; 78 percent Hispanic) -- 50,453 votes, Clinton 69%, Obama 29%

Polls in Texas are about to close, but we got some vote totals earlier because Texas is one of those states that allow "early voting." (The process is exactly as it sounds: voters are permitted to cast ballots ahead of the statewide primary election).

About 97,000 early votes have been counted in Travis County, which includes the state capital of Austin and the mammoth University of Texas campus. This was supposed to be an Obama stronghold, and he's beating Clinton there by 65 percent to 34 percent.

A different story in Bexar County, which includes San Antonio and is majority-Hispanic. More than 100,000 early votes have been counted and Clinton is leading Obama 54 percent to 45 percent. (According to exit polls, Clinton led Obama by about 30 percentage points among Hispanic voters.)

Because of a court order, we're not going to get any results of returns from Ohio until after 9 p.m. eastern time, so let's take a look at where the bulk of the vote will come in that state. Here are the 10 most populous counties in Ohio, according to 2006 estimates from the Census Bureau. These 10 counties include more than 6 million people, or more than half of Ohio's 11.5 million residents:

Cuyahoga (1,314,241) - Cleveland Franklin (1,095,662) - Columbus Hamilton (822,596) - Cincinnati Summit (545,931) - Akron Montgomery (542,237) - Dayton Lucas (445,281) - Toledo Stark (380,575) - Canton Butler (354,992) - north of Cincinnati Lorain (301,993) - Elyria, Lorain, Oberlin College; located west of Cleveland Mahoning (251,026) - Youngstown

The Clinton-Obama race is getting the lion's share of attention, of course, but we're also keeping an eye on how well John McCain does tonight in Ohio, Texas, Vermont and Rhode Island -- and whether he will clear the "magic number" of 1,191 delegates needed to clinch the Republican presidential nomination (campaign manager Rick Davis said Feb. 13 that his goal was to clinch today). McCain has already won in Vermont, where he won all 17 delegates that were at stake in that "winner-take-all" primary.

The networks have also declared McCain the winner in Ohio, where he won all 31 "at-large" delegates that were at stake. The winner of each of Ohio's 18 congressional districts wins all of that district's delegates. McCain will probably beat Mike Huckabee handily in Ohio, so it's possible that McCain will win all 18 congressional districts and sweep all of the Ohio Republican delegates.

McCain becoming the Republican presidential nominee is a matter of "when" and not "if." But he would like to be recognized as the official Republican nominee as soon as possible; Republican strategists surely would like the contrast between a Republican Party that has rallied behind their standard-bearer and a Democratic Party that may need more time to determine its nominee.

Some thoughts about the Ohio Democratic exit poll, as we await returns from Ohio's 88 counties ...

As has been the case in previous Democratic primaries, Clinton is doing well among lower-income voters and those with less formal education, while Obama is doing better among upper-income voters who are well-educated. Among those with no college degree (63 percent of respondents), Clinton led Obama 55%-44% in the exit poll. Obama led by that same margin among college graduates, though this bloc comprised just 37 percent of the vote. That seems to be a good sign for Clinton, notwithstanding the fact that the race appears too close to call at this very early stage.

Asked if the candidate had a "clear plan" for the nation, 68 percent said this was true about Clinton and 57 percent said this was true about Obama.

One of the most striking findings in the exit poll is that 78 percent of respondents said they were worried about their family's financial situation; this group favored Clinton, 52%-47%. Obama narrowly beat Clinton, 50%-49%, among the one-fifth of Ohio Democratic voters who said they were not worried about their family financial situation.

About 47 percent of Ohio Democrats said the top quality in a candidate is that he or she "can bring change," and these voters went for Obama 73%-27%. Among the 28 percent of respondents who said the top candidate quality is "experience," Clinton trounced Obama, 94%-6%.

No sooner had the big hand reached the "6" or the "VI" than the networks projected Ohio, where the polls closed at 7:30 p.m. eastern time, for John McCain. No surprise there. And in the Democratic race, they projected ... a close race between Clinton and Obama.

Barack Obama and John McCain are the projected winners in Vermont, where polling stations closed at 7 p.m. eastern time. This was probably the easiest call of the evening; Obama was strongly favored to prevail in Vermont, which is left-leaning and sparsely populated. McCain has always done well in the northeastern United States and Mike Huckabee was never going to seriously challenge McCain in Vermont.

Polling stations close in Ohio at 7:30 p.m. Stay tuned ...

We're starting to get some early exit poll data from the four states that are holding presidential primaries today -- Ohio, Texas, Vermont and Rhode Island. Some highlights: according to the Associated Press, six in 10 Ohio Democratic voters said that the economy was the number one issue; that's the largest share of voters who identified the economy as their paramount concern in any state Democratic primary for which exit poll data are available. And eight in ten Democratic voters said that international trade pacts are more harmful than helpful -- more compelling evidence that Ohio Democrats blame NAFTA and other trade deals as contributing to a hemorrhaging of U.S. jobs abroad.

By contrast, in left-leaning and dovish Vermont, where Barack Obama is expected to defeat Hillary Rodham Clinton, about as many Democratic voters identified the Iraq War as the top issue as did those who chose the economy.

Stay tuned to this space this evening (and through tomorrow morning!) for analysis on the returns in these states.