(Adds Clinton, Obama responses)

By Andy Sullivan

WASHINGTON, March 14 (Reuters) - Democrats in Michigan and Florida on Friday floated new plans to resolve the disputed state primaries that threaten to divide the party and its two presidential candidates well into the summer.

Several Michigan Democrats suggested re-staging their primary in early June while Florida Democratic Sen. Bill Nelson proposed letting the results of his state's contest stand but only seating half of its delegates at the August convention.

Both plans aim to avoid a standoff that threatens to mar the convention and increase bitterness between candidates Hillary Clinton and Barack Obama.

The January votes in Michigan and Florida were deemed invalid by the national party because both states had moved their election dates forward in defiance of party rules.

Clinton, a New York senator, won both primaries, though Obama's name was not on the ballot in Michigan. Both candidates avoided campaigning in either state.

Clinton has called for seating the delegates from both contests, or staging the primaries again. Obama, an Illinois senator, has said he will work to find an agreeable solution, but opposes seating delegates from the unsanctioned contests.

A solution has become vital as the rivals scratch for delegates in their tight battle for the right to face Republican John McCain in November's election.