Comcast Boston has refused to take down an ad attacking U.S. Rep. Charles Bass, R-N.H., for his support of a Republican plan to change Medicare. The National Republican Congressional Committee sent the cable television provider a letter arguing the Democratic commercial is false, The Washington Post reported Friday. The ad says the Republicans want to \"end Medicare.\" \"We are continuing to air the ad,\" Chris Ellis, a spokesman for the Comcast ad sales division, told the Post. \"We\'ve reviewed the materials provided by the NRCC and the documentation provided by the advertisers, and we\'ve decided that the ad does meet our guidelines.\" Republicans have already lost one special election in New York, largely because of the Medicare issue. House Minority Leader Nancy Pelosi, D-Calif., in an interview with ABC News, said the issue could restore control of the House to the Democrats in 2012. The plan, put forward by Rep. Paul Ryan, R-Wis., would convert Medicare from government insurance to vouchers the elderly could use to buy private insurance. Bass lost his seat in the House in the Democratic sweep in 2006. In 2010, he won a close contest to get it back.