Actor and former Tennessee senator Fred Thompson, is expected to annouce a bid to run for President.</p> <p>Thompson has been competitive with the top-tier trio in national popularity polls, which largely measure name recognition early in the campaign. Washington - Actor and former Tennessee senator Fred Thompson, is expected to annouce a bid to run for President.

Thompson has been competitive with the top-tier trio in national popularity polls, which largely measure name recognition early in the campaign. He is well-known nationally for his acting role as the gruff district attorney Arthur Branch on NBC’s long-running drama “Law & Order.”

His expected entrance in the race could dramatically shake up the GOP field but it’s unclear exactly who among the top GOP candidates - Giuliani, McCain or Romney - would be affected the most by his candidacy.

Conservative voters aren’t sold on any of them, which may give Thompson an opening. He’s been casting himself as a straight-talking conservative in the mold of former President Reagan even though he took some positions that angered the base as a senator.

Thompson served in the Senate from 1994 to 2002, capturing the seat held by Al Gore in a special election. During his tenure, Thompson voted to oust President Clinton from office, but also was one of 10 Republican senators who voted against one of the two impeachment charges.