Former Rep. Anne Northup (R-Ky.) will file this afternoon to reclaim the Louisville-based seat that she held for five terms before losing it to now-Rep. John Yarmuth (D) in 2006.

Northup will make her formal announcement at 1 p.m. today — the last day to file for office in Kentucky — at the secretary of state’s office in Frankfort, according to Ted Jackson, Northup’s longtime campaign chairman.

Yarmuth had been favored to win a second term in the Louisville district, which has given narrow victories to Democratic candidates in recent presidential elections.

But even though Democrats expressed confidence that Yarmuth will win again, Northup’s entry could make the race more competitive. A recent National Republican Congressional Committee-sponsored survey showed Northup polling within the margin of error in a hypothetical rematch with Yarmuth.

Jackson said that after continuing to ponder a rematch over the weekend, that poll helped finally sell Northup on another race.

“The poll is very strong and very compelling,” Jackson said. Northup was “very, very encouraged by these early numbers against Yarmuth.”

Jackson added that “there really is nobody else that could carry the banner and could step into this race at this late date and be more credible than Anne. … She believes in this, she believes in this district, she believes our party needs to be represented.”

The 3rd district leans Democratic in presidential elections — Sen. John Kerry (D-Mass.) took 51 percent of the vote there in 2004 — but until the previous cycle, Northup always was able to withstand the Democratic onslaught against her.

Northup’s entry into the race comes after the presumed Republican frontrunner, former assistant U.S. attorney Erwin Roberts, ended his bid two weeks ago after he was called up for a 12-month tour by the Army, where he serves as a captain in the Judge Advocate General’s Corps.

Roberts is a 35-year-old attorney who resigned in 2006 from his position as personnel cabinet secretary for then-Gov. Ernie Fletcher (R), who was embroiled in a hiring controversy. Last fall, Roberts received fundraising support from Northup, who lost to Fletcher in the Republican gubernatorial primary in the spring. Fletcher was soundly defeated in November by now-Gov. Steve Beshear (D).

Though Northup’s previous two campaigns have resulted in defeats, Jackson said her efforts in those races have benefits that will pay off on Election Day.

For one, Yarmuth is a freshman Congressman who never held elected office before defeating Northup. Meanwhile, the five-term former Congresswoman will be running her third high-profile race in two years in Louisville.

“Anne Northup’s name ID is much greater in this district than John Yarmuth’s,” Jackson said. “The governor’s race last year was important because it kept her name [in the media] plus Fletcher was a complete disaster. So Anne’s message [that Fletcher was unelectable] proved to be exactly right. … There was no net loss for her in losing that race.”

Although anti-tax activist and former professional football player Chris Thieneman (R) already is in the 3rd district race, a spokesman for the NRCC — where Northup’s former chief of staff, Terry Carmack, serves as political director — said Northup “would certainly be welcomed into the race.”

In an interview with a Louisville radio station on Monday, Thieneman, a wealthy real estate developer, complained that operatives from both Northup’s and Senate Minority Leader Mitch McConnell’s (R-Ky.) camp tried to get him out of the race last week to clear the road for the former Congresswoman. He promised to stay in the race and fight a vigorous grass-roots primary campaign.

Regardless of how vicious the primary turns out to be, it appears that one thing national Republicans will target Yarmuth on is his votes on illegal immigration — much the same way that the NRCC and conservative groups hammered Democratic special election candidate Robin Weirauch in her losing campaign in Ohio’s 5th district in December. Republicans say that issue in particular helped bring now-Rep. Bob Latta (R) success in the rural district.

“John Yarmuth has gone beyond being a rubber stamp for Nancy Pelosi, he crossed the line on a number of issues, including co-sponsoring legislation that would provide taxpayer-funded benefits for illegal immigrants and devastate the largest employer in his district,” said NRCC spokesman Ken Spain.

In a statement on Monday, Yarmuth shrugged off a Northup rematch.

“As long as I keep doing my job and I continue to receive the strong bipartisan support I’ve seen, my opponent won’t matter,” Yarmuth said.

Democratic Congressional Campaign Committee spokeswoman Kyra Jennings added that “voters in the 3rd district and across Kentucky rejected Anne Northup because she was wrong on priorities like protecting American jobs, giving tax breaks to companies that send jobs overseas instead of working families in the state. Northup was a champion for special interests, not for Kentucky’s middle class families, which is why she will be rejected again in 2008.”