TAMPA, Fla. - Heading into their nominating conventions, President Obama and Mitt Romney are dead even - down to a tenth of a percent - in the latest The Washington Times/JZ Analytics Poll, which also shows independents, the critical swing group, beginning to pick sides.

Mr. Obama and Mr. Romney both stood at 45.7 percent support, and Libertarian Party presidential nominee Gary Johnson garnered nearly 4 percent support. About 5 percent said they were undecided in an election looking like it will depend on which candidate can turn out his base supporters.

Voters are also increasingly enthusiastic about their pick, with independents in particular ditching their concerns and getting enthusiastically behind their chosen candidate.

"Both guys are consolidating their base and their base has now decided, 'Hey, this is our guy, he's good enough,'" said John Zogby, the pollster who conducted the survey of 800 likely voters Thursday through Saturday.

Republicans will nominate Mr. Romney later this week in Tampa, once their convention overcomes weather-related hiccups.

Democrats, meanwhile, will nominate Mr. Obama to seek a second term when they convene in Charlotte, N.C., next week.

The poll found a sizable gender gap, with Mr. Romney winning 54 percent of men and Mr. Obama winning 54 percent of women.

Just as important, however, is that Mr. Romney's lead among older voters has evaporated - though he has chipped away at Mr. Obama's lead among voters younger than 30.

The dead heat in this month's poll is consistent with other national polling. The previous two The Times/JZ Analytics Polls in May and July also found the race a dead heat, suggesting a stable contest.

For an incumbent to be polling less than 46 percent is worrisome for Mr. Obama, but more voters do still expect him to win the race, by an 11-point margin, over Mr. Romney, 48 percent to 37 percent. Republicans are increasingly optimistic over Mr. Romney's chances, but independents, who had been closely divided in the last survey, now give Mr. Obama a decided edge in the expectations game.

In an election that has become a web of charges and countercharges, voters say they can't trust either side to tell the truth.

Asked to rate the trustworthiness of Mr. Romney and his fellow Republicans on a scale of 1 to 5, with 1 being completely untrustworthy, the GOP team averaged 2.7 - the same as Mr. Obama and other Democrats averaged.

Republicans were inclined to believe Mr. Romney and Democrats were inclined to believe Mr. Obama, but independents were unimpressed with either. About half of independents rated Mr. Romney and Mr. Obama a 1 or 2 on the scale, meaning the voters believed little to none of what they said.

Mr. Zogby said Mr. Romney still has work to do to turn around those sorts of impressions during his nominating convention. But he said for Mr. Obama, the damage is probably done.

"Romney's got some selling to do. Minds are already made up about Obama, I think," he said.

The poll showed steady support for Mr. Johnson, the Libertarian Party nominee - but he did not build on his July number, suggesting he may have plateaued.

In order to secure a spot in the presidential debates, he will have to get at least 15 percent support in a cross section of polls, according to the rules laid down by the bipartisan commission that arranges the debates.

As for Mr. Obama and Mr. Romney, their supporters are warming to them. In July's survey, only 57 percent of Mr. Obama's supporters said they were voting for him because they thought he was the best man for the job - as opposed to simply being the lesser of two evils, or a vote against his opponent.

About 51 percent of Mr. Romney's supporters said he was the best pick.

But in the latest survey, 66 percent of each man's supporters now say they are backing their choice because he is the best candidate for the job.

The poll has a margin of error of 3.5 percentage points. In this poll, which was weighted for demographic purposes, those who didn't have a definitive choice for president were asked which way they leaned, and those were included in the head-to-head tally.

Data, Xtabs and Methodology for the Nationwide Poll