James Tobin, 45, one of three Republican campaign operatives convicted in a phone-jamming scheme designed to keep New Hampshire Democrats from voting in a 2002 election, was convicted of two telephone harassment charges.
Prosecutors had asked for a two-year sentence.
Steven McAuliffe. a US district judge, described the crime as extremely serious and a threat to free and fair elections.
"People in your position need to know they cannot do these things and if they do the consequences are very, very serious," he said in handing down a sentence harsher than the six months home detention and community service sought by Tobin's lawyer.
Democrats want an investigation into 22 telephone calls made by Tobin and New Hampshire Republican party officials to the White House on November 5-6, 2002, and say they believe national Republican officials may be involved in the scheme.
Kathleen Sullivan, the New Hampshire Democratic party chairperson, said she did not consider the sentencing to be the end of the matter.
"I consider this to be one more step in the process of uncovering exactly who knew about this. There are still unanswered questions," she said.
The national Republican party, which has paid more than $2.5 million in legal fees to defend Tobin, has said the calls to the White House were routine during a tight state Senate race and had nothing to do with the phone-jamming.
Get-out-the-vote hotlines set up by state Democrats and a firefighter's association to urge residents to vote were jammed by more than 800 hang-up calls.
State Republican officials say they tried to stop it once they learned of the scheme.
John Sununu, the Republican candidate, beat Jeanne Shaheen, the then governor in the election and state Republicans swept a number of close polls.
Tobin, the former New England regional director of the Republican National Committee, stepped down as New England chairman of Bush's 2004 re-election campaign when he became subject of a federal criminal investigation.
Several other senior Republican figures have been convicted for their parts in the scheme.
Tobin was denied bail and also fined $10,000 and given two years of probation. He plans to appeal, his lawyers said.