WASHINGTON — FedEx Corp., United Parcel Service Inc., and General Dynamics Corp. are among companies shifting campaign contributions to Democrats after years of favoring Republicans, federal records show.

FedEx’s political action committee contributed 58% of its money to Democrats in the first half of the year as it lobbies against a bill making it easier for the company’s truck drivers to unionize, according to the Federal Election Commission. In the 2006 election, two-thirds of FedEx’s PAC donations went to Republicans in their unsuccessful bid to keep congressional control.

Former FEC general counsel Larry Noble said corporate committees want to ensure they have a voice with the Democratic majorities in both the House and Senate. “They’re going to try to work with the Congress that exists,” Mr. Noble said. “What they are looking for is to make sure they are still heard.”

Eleven of the 20 leading corporate-PAC contributors gave more to Democrats than Republicans in the first half of 2007, FEC records show. Of the remaining nine, all but one increased the percentage of donations to Democrats. Each of the 20 top corporate political action committees in the 2006 election gave a majority of money to Republicans.

The switch in giving will be a boost for Democrats as they campaign to retain their majorities in 2008. Business PACs gave 65% of their money to Republicans in the 2006 elections, according to the Center for Responsive Politics in Washington, which tracks political giving.

Memphis-based FedEx said the donations aren’t tied to fighting the pro-union legislation. FedEx, the second-largest American package-shipping company, opposes a bill to let its truck drivers join local unions rather than having to organize nationally. The House Transportation Committee in June attached the provision to legislation renewing airline taxes and fees.

The measure would put FedEx truck drivers under the same labor law as those working for its larger competitor, UPS, which employs 246,000 Teamsters.

FedEx spokeswoman Kristin Krause said the donations are unrelated to the legislation. “We’ve always been very bipartisan in our PAC contributions and look for candidates on both sides of the aisle that understand our issues,” she said.

General Dynamics, the largest supplier of armored vehicles to the American military, also switched. The Falls Church, Va.-based company gave 58% of its political action committee donations to Democrats this year, compared with 36% in the 2006 elections, when Republicans were in control and wrote Pentagon spending legislation.

“We provide PAC contributions to those members who have like ideologies, basically strong for national security,” company spokesman Kendell Pease said.

Some companies increased donations to Democrats even though they pursue legislation the companies oppose.

The House in January passed legislation requiring Medicare to negotiate prices with drug companies, a measure that was blocked in the past by Republicans. New York-based Pfizer Inc., the world’s biggest drug-maker, gave Democrats 49% of its PAC donations in the first half of this year compared with 69% for Republicans in the previous election.

“Through political contributions, we express support for candidates who understand the importance of innovative life-sciences companies,” Pfizer spokesman Jack Cox said.

UPS boosted its Democratic giving this year to 40% from 32% for the 2006 elections. “We support candidates who share our beliefs on fair trade and fair competition,” UPS spokesman Malcolm Berkley said.

Atlanta-based UPS has given more money to federal candidates than any other company PAC in each of the last eight elections. It was overtaken by San Antonio-based AT&T Inc. during the first six months of 2007. AT&T, which acquired BellSouth Corp. last year, increased the money going to Democrats from 30% to 41% so far this year.

AT&T and its allies last year beat back a provision that would have prevented them from charging Web sites for priority access to their high-speed networks. The measure was supported by Rep. John Dingell, a Democrat of Michigan, who’s now chairman of the House Energy and Commerce Committee, and Rep. Edward Markey, a Democrat of Massachusetts, who heads the panel’s telecommunications subcommittee. AT&T spokesman Mike Balmoris didn’t respond to requests for comment.