Ray Linsenmayer announced his decision Sunday, four weeks before the May 15 primary, and is urging his supporters to back Lamb.
Rothfus has no primary opponent in the newly drawn 17th District west of Pittsburgh.
Lamb is currently serving the remaining nine months in the term of now-resigned Republican Rep. Tim Murphy in southwestern Pennsylvania.
The boundaries of Pennsylvania's congressional districts are changing with the November election. That's because Pennsylvania's Supreme Court ordered new boundaries to replace the 6-year-old boundaries that it ruled were unconstitutionally gerrymandered by Republicans.
The new 17th District is considered a tossup.
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