Twenty-three South Korean lawmakers have left the ruling Uri party to form a new political group to fight elections later in the year.
President Roh has seen his popularity fall sharply
The move leaves the conservative opposition Grand National Party (GNP) as the largest group in parliament.
Correspondents say the breakaway MPs want to distance themselves from the increasingly unpopular President Roh Moo-hyun ahead of December elections.
A recent opinion poll put the Uri party's approval rating at just 10%.
In contrast, the GNP is enjoying public support of around 50%, the same poll found.
In a statement, the 23 politicians said they would continue to co-operate with President Roh during his remaining 10 months in office "but we strongly reject any political intervention".
"With our deep regret and apology to the public, which is against the Uri Party, we are determined to give up our current power and become a seed to form a new united party," one of the defectors, Lee Jong-gul said at a press conference.
Their move leaves the Uri Party with just 110 seats in the 299-member National Assembly. The GNP has 127 seats.
Mr Roh had appealed to party members to stick together to fight the GNP in December's presidential election, and had even offered to resign from the party.
But the 23 lawmakers follow several other Uri Party members who have left in recent weeks, and more defections are expected.
Political analysts say the defections are likely to split the Uri Party, which was itself created as a splinter from the then ruling Democratic Party just three years ago.
Both the party and its president have seen their popularity plummet amid increasing public dissatisfaction over a stagnant economy.