By Mark Hosenball
WASHINGTON (Reuters) - Republicans are pouring more last-minute cash into tight Senate races than their rivals as some opinion polls show Democrats may not do as badly as expected in Tuesday's congressional elections.
Data compiled by a non-partisan election watchdog and Democratic organizers shows substantial late campaign spending by pro-Republican groups on Senate contests in Colorado, Kentucky and California.
Those three states are among the 10 that Republicans must win to gain control of the 100-seat Senate.
Figures from the Wesleyan Media Project, an academic group that tracks political advertising in every U.S. media market, show roughly equal spending by Republicans and Democrats in other important Senate races, including Washington state and Nevada.
Americans will vote for 37 Senate seats and all 435 seats in the House of Representatives, as well as state governors and lawmakers and local officials.
President Barack Obama, a Democrat, is not up for re-election until 2012 but Republicans are casting Tuesday's vote as a referendum on his leadership as the economy recovers slowly from recession and unemployment stays high.
Experts expect Republicans to win control of the House and to gain seats in the Senate but not enough to wrest control from the Democrats.
A surge in spending is typical in the final days of U.S. election campaigns, often targeted based on last-minute poll results. But some non-partisan election monitors are skeptical about how much influence the deluge of spending and advertising will have on voters.
Veteran pollster Andrew Kohut of the Pew Research Center said most voters' minds are already made up by now.
"There is such a glut of material (that) a lot of it is going to get lost," he said.
Figures given to Reuters by Democratic organizers show that, from August through late October, independent Republican political committees -- groups not affiliated with official local and national party organizations -- outspent Democratic rivals in Senate contests by a ratio of more than 3-to-1.
While Democrats have received less money from these independent groups, experts say they made up the difference with funds from official local and national party organizations throughout the year.
SPENDING IN TIGHT RACES
* In Nevada, where polls show Senate Democratic leader Harry Reid in a dead heat with Republican challenger Sharron Angle, late campaign spending is running roughly equal, according to the Wesleyan and Democratic organizers' figures.
* In Colorado, where polls show incumbent Democrat Michael Bennet closing the gap on conservative Tea Party challenger Ken Buck, Republican groups have poured in last-minute funding by a ratio of more than 2-to-1 over Democrats.
* In Kentucky, where polls show Tea Party favorite Rand Paul has lost some ground to Democrat Jack Conway, last-minute pro-Republican spending has been higher than groups backing Conway by a 2-to-1 margin.
* In California, Wesleyan's figures show incumbent Senator Barbara Boxer had a more than 2-to-1 funding advantage over Republican challenger Carly Fiorina through her own and other official Democratic Party groups. But pro-Fiorina groups not affiliated with official party organizations have outspent pro-Boxer groups by a ratio of at least 5-to-1.
* In Pennsylvania, Democratic figures show pro-Republican groups greatly outspending pro-Democratic groups late in the campaign. Polls in Pennsylvania show Democrat Joe Sestak closing in on the one-time heavy Republican favorite, former Representative Pat Toomey.
* In Delaware, figures from Democrats show Republican Senate candidate Christine O'Donnell, whose official campaign has raised substantially more than Democrat Chris Coons, has received a tiny amount of support -- totaling around $300,000 -- from right-wing and pro-Tea Party groups.
The figures show that even though Coons trails O'Donnell in official campaign cash, he has had no additional support from independent pro-Democratic groups -- a reflection of the assessment that O'Donnell's consistently poor polling numbers suggest she will probably lose on Tuesday.
(Editing by John O'Callaghan)