U.S. TopoJSON

These county, state and country boundaries are extracted from a single TopoJSON file from us-atlas. See also the non-fancy variant.

<!DOCTYPE html>
<svg width="960" height="600" stroke-linejoin="round" stroke-linecap="round">
  <defs>
    <filter id="blur">
      <feGaussianBlur stdDeviation="5"></feGaussianBlur>
    </filter>
  </defs>
</svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script src="https://d3js.org/topojson.v2.min.js"></script>
<script>

var svg = d3.select("svg");

var defs = svg.select("defs");

var path = d3.geoPath();

d3.json("https://d3js.org/us-10m.v1.json", function(error, us) {
  if (error) throw error;

defs.append("path")
      .attr("id", "nation")
      .attr("d", path(topojson.feature(us, us.objects.nation)));

svg.append("use")
      .attr("xlink:href", "#nation")
      .attr("fill-opacity", 0.2)
      .attr("filter", "url(#blur)");

svg.append("use")
      .attr("xlink:href", "#nation")
      .attr("fill", "#fff");

svg.append("path")
      .attr("fill", "none")
      .attr("stroke", "#777")
      .attr("stroke-width", 0.35)
      .attr("d", path(topojson.mesh(us, us.objects.counties, function(a, b) { return (a.id / 1000 | 0) === (b.id / 1000 | 0); })));

svg.append("path")
      .attr("fill", "none")
      .attr("stroke", "#777")
      .attr("stroke-width", 0.70)
      .attr("d", path(topojson.mesh(us, us.objects.states, function(a, b) { return a !== b; })));
});

</script>

https://d3js.org/d3.v4.min.js

https://d3js.org/topojson.v2.min.js