Jovair Arantes, rapporteur for a special impeachment commission in the lower house of Congress, said he had concluded the "legal admissibility" of the case against the leftist president.
The decision was given in a lengthy report that Arantes read aloud, live on national television, to the 65-member impeachment commission, sometimes interrupted by deputies shouting and arguing.
Although Arantes' decision was non-binding and mostly of symbolic value, it meant the opposition drew first blood in a lengthy and increasingly bitter battle to remove Brazil's first woman president from office.
She is accused of presiding over large-scale fiddling of government accounts to mask the depth of budgetary shortfalls during her reelection in 2014.
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Rousseff -- highly unpopular because of a severe recession and a giant corruption scandal enveloping Brazil's political elite -- says she has committed no impeachment-worthy crime and claims she is the victim of a coup attempt.
Intrigue is rife over which way Congress will lean on the 18th.
Yesterday, the murky political landscape entered extraordinary new territory when a Supreme Court judge ruled in favour of a bid to also impeach the vice president, Michel Temer, who has become a leading opponent of Rousseff -- and would replace her if she had to step down.
In the impeachment request, Temer is accused of participating in the same fiscal juggling as Rousseff.
Although proceedings against Temer are highly unlikely to get underway soon and could still be thrown out by the full Supreme Court, the judge's ruling weakened the opposition camp.
Her Workers' Party is now scrambling with the help of smaller allies to build a new coalition.