The Republican party showed signs of disarray today over the prospect of a Barack Obama presidential victory being matched by sweeping gains on November 4 by the Democrats in Congress on a scale not witnessed since the 1930s.

Figures show the Democrats outspending the Republicans 4-1 in congressional races across the country. Democrats are also being helped by huge turnouts of African-Americans and young voters in states where Obama is unlikely to win but where those in Congressional races stand to benefit.

Among indications of panic today, the National Republican Congressional Committee pulled $50,000 (£31,000) in ads scheduled for broadcast from Monday for Michele Bachmann, a Republican congresswoman from Minnesota. The party effectively disowned her over remarks she made on television on Friday in which she said Obama "may have anti-American views". The committee is also withdrawing funding from other congressional races seen as futile contests.

If Obama won the White House and the Democrats gained more seats in the House and Senate - and even reached the magic 60 seats in the Senate that would allow the Democrats to override any Republican attempts to filibuster legislation - he would potentially be in one of the strongest positions enjoyed by any Democratic president since Roosevelt in 1932.

Tom Mann, a political analyst at the Brookings Institution, said today the Democrats could gain seven to 10 Senate seats and 20 to 30 in the House.

With polls showing Obama extending his lead in key states, John McCain fought today on a "Joe the plumber" platform, saying an Obama presidency would mean tax increases for the working and middle classes. He also returned to the theme that Obama is too inexperienced to face an international crisis.

Obama, after a rally in Indianapolis, abandoned campaigning temporarily to visit his ailing grandmother, Madelyn Dunham, in Hawaii. In a CBS interview, Obama said he did not want to repeat the mistake of not being at his mother's bedside before she died. He said of his grandmother: "We knew she wasn't doing well, but you know, the diagnosis was such that we thought we had a little more time and we didn't."

The Republican party is facing tight races across the country in congressional seats it would normally regard as safe. Even Elizabeth Dole, a Republican senator from North Carolina, which has been Republican for 35 years, is on the defensive, with an ad warning voters that the Democrats should not be allowed total control of the White House and Congress. The Democrats had struggled to find someone to stand against her, given she was regarded as unassailable, but the eventual candidate, Kay Hagan, now enjoys a narrow poll lead.

In Minnesota, Bachmann's Democratic rival, Elwyn Tinklenberg, had also been regarded as a no-hoper. But since Bachmann's remarks about Obama, Tinklenberg has been the beneficiary of a backlash, taking in $1.3m in donations since Friday.

The non-partisan Campaign Finance Institute reported yesterday that the Democratic Congressional Committee has spent $37m since August on behalf of its congressional candidates, compared with the NRCC's $9.6m.

Democrats are expected to make gains in the Senate in southern states such as Mississippi, Kentucky and Georgia, where there are big African-American populations, many of whom are set to turn out for the first time to vote for Obama.

• This article was amended on Friday October 24 2008. We erroneously said Michele Bachmann's surname was Bachman. This has been changed.