WASHINGTON — President Barack Obama vowed today to veto a financial overhaul bill that doesn't regulate the eclectic derivatives market, even as Senate Republicans lined up en mass against it.

Legislation pending in Congress would for the first time regulate derivatives, complex financial instruments like the mortgage-backed securities that contributed to a near economic meltdown in 2008 when their value plummeted during the housing crisis.

Obama said he wants derivatives to be strongly regulated, and he added that he's ready to veto any financial reform bill that comes to his desk without it.

But there's some dispute among Democrats about how far such regulation should go.

Further complicating matters today, Senate Minority Leader Mitch McConnell obtained signatures from 40 fellow GOP senators voicing opposition to the Senate bill and demanding further negotiations.