PORTLAND – Democratic Rep. Tom Allen filed papers Tuesday formalizing his challenge to two-term Sen. Susan Collins, a race shaping up as one of a handful likely to draw the national spotlight in 2008.

Allen sought to cast the election as a referendum on the Iraq war, which he called “the worst foreign policy mistake in our nation’s history.” He blamed Collins for voting to support the war and to oppose a timeline for bringing the troops home.

“From the beginning, when President Bush rushed to invade Iraq, Susan Collins has supported his misguided policy. I fought to stop it. She voted for the Iraq war. I voted against it. Susan Collins continues to vote with the Republicans against a timetable to end the war in Iraq,” he said in an online video announcement.

Allen, a six-term Democrat from southern Maine’s 1st District, filed Federal Election Commission paperwork and launched his campaign Web site. He said duties in Washington forced a delay in his return to Maine for a formal announcement, which likely will happen during the Memorial Day recess.

His entry into the race coincided with release of the first independent poll, which showed Collins leading by 25 percentage points among likely voters. The Critical Insights telephone poll of 600 respondents had a margin of error of plus or minus 4 percentage points.

Allen said he was neither surprised nor worried by the poll, which showed Collins with 57 percent, Allen 32 percent and 11 percent “undecided” or “don’t know.”

“We’re not doing this because of polls,” Allen said in a telephone interview. “Maine voters are going to have a chance to engage in the national debate about the direction of this country. We’ve had six years of a Republican administration with strong support from Susan Collins and a Republican Congress taking us in a direction that I think has been disastrous.”

The Allen-Collins race is seen as an indicator of whether moderate Republicans such as Collins and fellow Maine Sen. Olympia Snowe have a future in regions like the Northeast that seem to be turning bluer with each national election.

Along with contests in Colorado, Minnesota and New Hampshire, the race has begun showing up on virtually everyone’s watch list during the coming Senate cycle.

“It will be a test as to whether the Democratic tide in New England is continuing or if it crested in 2006,” said John Pitney, a professor at Claremont McKenna College in California and former research analyst for Republicans. “The tide has been strong and if it can take down [former Rhode Island Sen.] Lincoln Chafee, it may take down Susan Collins as well.”

The race also is shaping up as Maine’s most expensive ever.

Collins is off to a fast fundraising start, having taken in $832,075 in the first quarter of this year. That, according to her filing with the Federal Election Commission, raises the amount of cash available to her campaign to $1.2 million.

Allen raised $393,243 in the quarter to boost his cash on hand to $812,484.

With more than a year and a half to go before the election, television spending already has begun. Collins’ foes made the first big advertising buy of the campaign when they spent $100,000 in early April to air a 30-second television spot criticizing her opposition to a deadline for withdrawing U.S. troops from Iraq.

Allen supporters were continuing to hammer away at that theme. Collins is among four Republican senators targeted in a series of TV ads scheduled to begin Wednesday in which two retired Army generals who served in Iraq oppose Bush’s stance on the war.

In her bid for a third term, Collins brings high approval ratings, a solid campaign war chest and a long-standing reputation for working across party lines.

Her high-profile role as chairwoman, and now ranking Republican, of the Senate Homeland Security and Governmental Affairs Committee has put her in the forefront of such issues as port security and the recovery from Hurricane Katrina.

She campaigned for the committee’s top Democrat, Joseph Lieberman, during his bruising re-election bid as an independent last November, and the Connecticut senator already has said he plans to return the favor on behalf of Collins.

Meanwhile, Ned Lamont, who defeated Lieberman in the Democratic primary before losing the general election, said he intends to team up with Massachusetts Sen. John Kerry to try to unseat Collins and three other GOP senators.

Allen, a former Rhodes scholar who studied at Oxford with Bill Clinton, is a former Portland mayor who lost in a five-way primary for the Democratic gubernatorial nomination in 1994. He was first elected to the House two years later and has won re-election easily ever since.

His plans this year became evident early on. Already, at least four Democrats have signaled their intentions to run for his House seat.

Democrats are taking heart from last November’s defeat of Chafee, one of the dwindling corps of GOP moderates. Like Collins, he was popular and had high approval ratings, noted Allen’s campaign manager, Valerie Martin.

The Collins camp says the senator’s centrist position and willingness to cross party lines present a clear contrast to Allen’s more partisan approach.

“Mainers will quickly draw the distinction between Senator Collins and Congressman Allen in everything ranging from their accomplishments to their small-business records. Senator Collins has demonstrated her effectiveness by working across party lines for the benefit of Maine and the nation,” said her spokeswoman, Jen Burita.