But the numbers illustrate a stark challenge for Republicans. In the House, 23 seats held by Republicans are generally rated tossups and 4 others are leaning Democratic; just 8 Democratic seats fall into the tossup category and one, now held by Representative Nick Lampson in the Houston area, is rated likely to change parties. In the Senate, one Republican seat  Virginia  is considered safely in the Democratic column, and Alaska and New Mexico are considered leaning Democratic. Five states  Colorado, Minnesota, New Hampshire, North Carolina and Oregon  are tossups.

“We’re doing extremely well in places we didn’t expect to do well,” Senator Charles E. Schumer of New York, chairman of the Democratic Senatorial Campaign Committee, said Wednesday.

In putting their vying messages before the voters, the Democrats’ campaign committees have an edge in how much advertising they can afford. The committees tend to focus their spending in the most competitive districts.

The National Republican Congressional Committee has spent well under $1 million on advertisements in House districts, compared with more than $16 million invested in advertising by the House Democrats’ campaign committee. And it can only afford to spend in defense of select Republican seats. On Wednesday, the Capitol Hill newspaper Roll Call first reported that the Republican campaign group was borrowing $8 million to buy more advertising in the closing weeks, and to avoid being heavily outspent.

In district after district, House Democrats are running advertisements seeking to link Republicans with President Bush and his economic record.

A Democratic commercial against Representative Christopher Shays of Connecticut, the only Republican House member in New England, shows him shaking hands with Mr. Bush. It includes audio of a September radio interview where Mr. Shays asserted that “our economy is fundamentally strong.” A similar statement by Mr. McCain put him on the defensive last month. Polls show Mr. Shays with a lead, but Democrats say their candidate, Jim Himes, is gaining ground.

Mr. McConnell, the Republican leader whom Democrats would relish knocking off as payback for the 2004 defeat of their leader, Senator Tom Daschle of South Dakota, is being tied to the economic deterioration in new commercials on behalf of his Democratic challenger, Bruce Lunsford, a wealthy businessman.