WASHINGTON — Since mid-April, the average price for a gallon of regular gasoline has dropped nearly 40 cents, President Barack Obama has announced support for same-sex marriage, government statisticians have delivered two disappointing monthly jobs reports, tensions have ebbed and flowed with Iran, and Mitt Romney has clinched the Republican presidential nomination.

And the presidential polls?

Flat-lined. Contradicting reams of punditry, national polls have not moved an inch amid those events — not to mention the lesser political battles that have animated cable news programs.

In Gallup’s daily polling, to take one example, Romney and Obama were tied 46 percent to 46 percent on April 11. Two months later, the poll had Obama up one point, 46 percent to 45 percent, a statistically identical result.

For more than seven weeks, neither candidate’s standing has moved more than 3 points — well within the poll’s margin of error. Instead of a race, the campaign for President has turned into something more closely resembling trench warfare: dug-in armies, intense exchanges of fire, no movement.

The lack of movement is problematic for Obama. Both candidates, of course, would like to have broken free by now. But for Romney, just keeping Obama below 50 percent counts as an advantage, on the assumption that a majority of late deciders are more likely to vote against the incumbent.

By contrast, many Democratic strategists had hoped that by now Obama would have started to build a lead over the Republican, whom they derided earlier this year as a weak nominee with little popularity even within his own party. The stasis reflects the electorate.

Over the last decade, voters have become polarized into warring partisan camps, more so than at any point since the 1930s. Obama and Romney each get nearly 90 percent support from their respective partisans, most of whom have strongly held views.

Meantime, pollsters report that the percentage of undecided voters hovers somewhere in the high single digits, with even fewer in some swing states.

“Two big things are going on,” said Stanley B. Greenberg, who served as the chief pollster for Bill Clinton’s campaign in 1992 and Al Gore’s in 2000.

The poor condition of the economy holds Obama’s vote down, he said. At the same time, demographic trends that favor Democrats — an increasing number of nonwhite voters and a greater percentage of college-educated professionals — pushes his vote up.

Those opposing forces have combined to lock the race into a nearly even division that has proved stubbornly resistant to change. “Obama right now is a 47 percent candidate” while “Romney is a 45 percent candidate,” Greenberg said, reflecting the findings of his own polls and several other independent surveys that show Obama with a slight lead.

“Structurally, it will probably stay there for some time.” Still, the election is nearly five months away, and in 2004, the last presidential re-election contest,

President George W. Bush did not establish a lead over Sen. John F. Kerry, the Democratic nominee, until August. (That lead took hold before the airing of “Swift boat” television ads attacking Kerry’s war record, which many Democrats have blamed for Kerry’s defeat.)