The ARD television exit poll put support for Merkel's Union bloc at 42 percent. But it put her coalition partners, the pro-business Free Democrats, on 4.7 percent - just below the minimum to keep their seats.
Challenger Peer Steinbrueck's Social Democrats won 26 percent and the Greens 8 percent, according to the exit poll. The hard-line Left Party won 8.5 percent.
The survey showed a new anti-euro party with 4.9 percent - uncertain of winning seats.
If Merkel's coalition lacks a majority, the likeliest outcome is an alliance with the Social Democrats.