The US Democratic party has tightened its grip on the senate after the Minnesota supreme court ruled that Al Franken, its candidate, won a disputed election more than seven months ago.

With Franken declared winner of the Minnesota seat and the usual backing of two independent senators, Democrats now have 60 seats in the 100-seat senate - a big enough majority to overcome possible Republican delaying tactics on legislative votes.

The Democrats already have a strong majority in the House of Representatives, the lower house of the US bicameral legislature, and increased control of the senate would aid Barack Obama, the US president, in advancing his legislative agenda.

Hours after the unanimous court decision, Norm Coleman, the Republican candidate, conceded to Franken on Tuesday, bringing an end to a nearly eight-month recount and court fight over an election decided by only a few hundred votes.

Franken, an author and former comedian appearing on the popular Saturday Night Live television programme, "received the highest number of votes legally cast and is entitled … to receive the certificate of election as United States senator from the State of Minnesota", the court ruling said.

The two candidates had been engaged in a months-long legal fight over the validity of the November 4 election.

A series of recounts in the state gave Franken a victory of just 312 votes over Coleman, prompting the Republican to challenge the accuracy of the recount.

Tim Pawlenty, the Minnesota governor and a Republican, is now expected to formally grant the senate seat to Franken.