Twelve Republican U.S. senators on Thursday sent a letter to Senate leaders criticizing pending patent legislation, saying the bill "threatens to diminish the value and enforceability of U.S. patent rights."

The Oct. 15 letter backs criticism against the legislation being levied by independent inventors and academics who argue the bills favor major technology companies. If approved the legislation would be the most sweeping rewrite of federal patent law in 50 years.

Critics say two similar bills now in Congress would broadly make it harder for individuals, universities and start-ups to defend their inventions against companies with deeper pockets. Opponents are also upset that Obama administration officials with past ties to International Business Machines Corp. (IBM) and Microsoft Corp. (MSFT) are supporting the bill.

At issue is a provision giving outside challengers expanded ability to initiate review of newly approved patents. "These so-called post-grant review provisions, as currently crafted, are quite problematic," the Republican senators said in the letter written by Sen. Sam Brownback, R-Kan., and signed by 11 other Republicans.

Microsoft and IBM are two of the most active companies involved in filing patents; IBM, for example, earns over $1 billion each year in intellectual property-related income. Both companies have ties to top government officials with authority over patent policy, including U.S. Patent and Trademark Office Director David Kappos.

In testimony before Congress last March, Kappos - who was, back then, the head of intellectual property at IBM - praised the post-grant review. Kappos, who spent 26 years at IBM, said in an interview Thursday that he recuses himself from IBM-specific matters and has taken pains since his appointment to listen to the concerns of independent inventors.

"If I wanted to do the right thing for a specific company, I could have stayed there," he said. IBM declined to comment.

Kappos also said he told lawmakers in private meetings they should establish a high threshold for reviews and include safeguards against serial challenges.

But even one post-grant review can cause an inventor to lose market advantage and venture capital funding.

"Patent certainty is critical to the ability of a venture capitalist to be able to invest," said Kelly Slone, director of the National Venture Capital Association's medical industry group. More than a dozen research universities have also sent letters opposing the post-grant review provision.

Microsoft, too, has connections in the Commerce Department. Marc Berejka, a senior official handling intellectual property matters, worked for Microsoft for the 12 years leading up to his July 2009 hiring, including eight years as a lobbyist for the company.

Commerce Secretary Gary Locke, who endorsed most of the bill in an Oct. 5 letter, received campaign donations from Microsoft employees as Washington state governor. Locke accepted $96,900 in cash donations from company employees while running for re-election in 2000, according to state public records. In total, Locke raised $3.8 million in that election cycle. He also owned between $100,000 and $250,000 of Microsoft stock until divesting it on June 22 as a condition of his appointment.

A Commerce spokesman said the agency is deferring to Congress on how to alter patent laws.

Pat Choate, an economist and board member of American Innovators for Patent Reform, said he was troubled by the process. "Patent policy is suddenly being run by advocates for Microsoft and IBM," Choate said.

But some representatives of small companies say they think the government is trying to work in good faith.

"It is certainly the case that big companies have a lot of very well-qualified people who work with them and then go and do other things," said Brian Pomper, executive director of Innovation Alliance.

-By Kristina Peterson, Dow Jones Newswires; 202-862-6619; kristina.peterson@dowjones.com