By Ted Hesson WASHINGTON (Reuters) – Republicans in the U.S. Congress are blocking emergency funding for Ukraine and threatening to force a government shutdown in an effort to tighten security along the U.S.-Mexico border. WHY ARE REPUBLICANS FOCUSING ON BORDER SECURITY? Record numbers of migrants have been caught illegally crossing the border since Democratic President Joe Biden took office in 2021. The U.S. Border Patrol arrested about 2 million migrants at the U.S.-Mexico border in fiscal year 2023, similar to record-breaking totals during Biden’s first two years in office. During Republica…