Unusual Attack Profile Needing Help

I have a site that I am webmaster for that is used by local law enforcement in Central Florida. While there is nothing confidential or private on the site, I have been using Cerber Plugin – Free Version with wonderful results. However, 2 days ago, the site began receiving regular attempts to log in using the default WordPress ID, admin. The attempts are successfully blocked. They come about 6 minutes apart and total around 700+ per day. Totals now exceeding 2,000. What is unusual is that every attempt is with a new IP address. The plugin stats show it comes from Malaysia:

FILTERED WHOIS INFO
inetnum 175.144.0.0 – 175.144.255.255
netname ADSL-STREAMYX
descr TMNST
country Malaysia (MY)
admin-c TIA7-AP
tech-c TIA7-AP
status ALLOCATED NON-PORTABLE
mnt-by TM-NET-AP
mnt-lower MAINT-AP-STREAMYX
mnt-routes MAINT-AP-STREAMYX
mnt-irt IRT-TMNST-MY
notify [email protected]

last-modified 2014-02-11T04:58:41Z
source APNIC
irt IRT-TMNST-MY
address Jalan Pantai Baru, Kuala Lumpur.
e-mail [email protected]

abuse-mailbox [email protected]

auth # Filtered
person EMRAN AHMED KAMAL
nic-hdl EAK2-AP
phone +6-03-83185434
fax-no +6-03-22402126

Each WHOIS retrieval shows a new IP address but the same data on the network. There does not appear to be any way to stop it other than relying on Cerber to do the initial block during the login attempt. Does anyone have advice? Is this a serious concern? The site is https://cfcpa.org/

Bob Samson