WASHINGTON — A record 22 Democratic candidates have raised at least $1 million each to challenge incumbents in the most competitive House races, another sign that Republican control of the chamber is in jeopardy in next month’s election.

The number who had passed the $1 million threshold by the end of last month was nearly double the number at a comparable point in 2004, according to reports filed with the Federal Election Commission in Washington.

“Often, the ability of a challenger to raise money is a sign that the incumbent is vulnerable,” a government professor at Dartmouth College in Hanover, N.H., Linda Fowler, said. “On average, challengers who beat incumbents spend about a million dollars.”

The pace of Democratic fund-raising is likely to increase in the wake of new polls showing the party has a strong shot at picking up the 15 seats that it needs to gain control of the House on November 7. A National Public Radio poll released Tuesday found that voters in 48 competitive House districts favor the Democrat over the Republican by 51% to 40%.

“One of the oldest fund-raising strategies in the books is to point to polling data that shows that congressional candidates are potentially viable,” the chairman of the FEC, Michael Toner, said. “There’s no question that the favorable polling data that Democrats now have has to be beneficial in their fund-raising.”

Eight of this year’s challengers in competitive races — seven Democrats and one Republican — had raised at least $2 million through September 30, FEC records show. No challenger had done so by that date in 2004, and the one who had in 2002 — Democrat James Humphreys of West Virginia, who was ultimately defeated — provided most of the money from his own pocket.

“This is a pretty good sign that the challengers are ready to take it to the incumbents,” Michael Malbin, who is executive director of the Campaign Finance Institute, a Washington-based research group, said.

All four of the Republican challengers in the 35 most competitive House races as ranked by three publications that monitor congressional elections had crossed the $1 million mark by September 30, FEC records showed. The publications are Congressional Quarterly, the Cook Political Report, and the Rothenberg Political Report.

Two of the Democrats who raised at least $1 million this year, Tim Mahoney in Florida and Nick Lampson in Texas, are running in districts where ethical problems forced the withdrawal of Republican incumbents — Mark Foley in Florida and Tom DeLay in Texas.

Even in those districts where Democratic challengers have raised $1 million, all but four still trail Republican incumbents in the money race. In Arizona, Rep. J.D. Hayworth, a Republican, raised $2.4 million through September 30, compared with $1.3 million by Democratic challenger Harry Mitchell. Mr. Hayworth also had more money available to spend during the last six weeks of the campaign — $1.3 million, compared with $565,866 for Mr. Mitchell.