In recent years, republicans who have failed to toe the party line have switched to the democratic party. have dominated leadership positions in congress. have most often been the most extremely conservative members of the party. have defeated many rinos. have faced serious challenges in the republican primary.

The correct answer is that they have faced serious challenges in the republican primary

Republican party elected members that have failed to toe the party line have often lost their reelection bids, to more conservative candidates. This has been a consequence of an electorate that has increasingly become polarized with a larger proportion of more conservative electorate participating in elections