Changing opinions, particularly among voters who label themselves independents, and the impact of campaign events – especially the first presidential debate – have brought both the presidential and US Senate races in Wisconsin to dead ties, according to results of a new round of the Marquette Law School Poll released Wednesday.

Both races are now pure toss-ups, said Professor Charles Franklin, director of the poll and visiting professor of law and public policy at the Law School.

In poll results four weeks ago, Democratic President Barack Obama led by 14 points over Republican challenger Mitt Romney. Two weeks ago, Obama was up by 11 points. But in polling done Oct. 11 to 14 – just ahead of the second presidential debate – Obama was favored by 49% and Romney by 48%, effectively a tie.

Franklin pointed to the impact of the presidential debate in Denver as a central factor. Obama’s performance in that debate was widely panned. “Rarely has a debate produced such a large movement in the polls,” Franklin said.

In the Senate race, former Gov. Tommy Thompson , the Republican candidate, was ahead by nine points in polling in mid-August. A month later, Rep. Tammy Baldwin, the Democratic candidate, was ahead by nine points. Now, the two are in a virtual tie.

Franklin said Baldwin surged during a period when Thompson’s campaign was not doing television advertising and her campaign was. Thompson’s launch of rounds of advertising appears to have brought Thompson back into a tie.

Franklin said there continues to be very strong loyalty to candidates by those who identify themselves that candidate’s party. But results have shifted among those who identify themselves as independents.

One more round of polling in the year-long Law School project is scheduled before the Nov. 6 election. For full results of the new polling, click here.